Applications for drive mechanisms requiring low speed and high torque such as in bicycles and other exercise equipment have generally used roller chains. Roller chains have their pitch line fixed by a line going through the center of their rollers which lies below the tips of the sprocket teeth. The surface of the sprocket tooth engages the roller at a point when it is perpendicular to the pitch line, thus, yielding a zero pressure angle. Therefore, when a torque is applied to the sprocket, the chain does not tend to ride off the sprocket even when the slack side of the chain has zero tension. In many instances, the chain will break before it climbs out of the sprocket teeth and ratchets.
In synchronous drive belt and pulley systems, such as illustrated in my prior U.S. Pat. Nos. 3,924,481; 3,969,946 and 4,108,011, the tensile load bearing component on which the pitch line is centered lies outside the pulley teeth. Those belts, generally for use in high speed, low torque applications, have their fore and aft tooth flanks designed to have a pressure angle between 10.degree. and 20.degree. to allow for smooth engagement and disengagement. The pressure angle is that angle at which the belt tooth and the pulley tooth flanks contact each other when compared to a line drawn radially from the pulley center. To minimize ratcheting, or the tendency for the belt to ride off the pulley sprockets, it is desirable to have a pressure angle of zero.
Another factor influencing ratcheting is the distortion of the tooth under load. Synchronous belt teeth are made from elastomeric materials and as such will deform under load. They are cantilevered on the belt tensile member, thus, as the belt tooth distorts over the pulley tooth, the pressure angle increases resulting in an outward or radial force tending to lift the belt out of the pulley. U.S. Pat. No. 3,756,091 illustrates a belt having a zero pressure angle. It can be appreciated, however, that the contact surface on the belt tooth flank at zero pressure angle is very short and the pressure angle increases rapidly along the flank away from the tooth root.
In accordance with the practice of the present invention, there is provided a synchronous drive belt wherein the belt teeth have a zero pressure angle over a relatively greater length along the tooth flank and wherein other design criteria as set forth herein have been specified to reduce tooth distortion. There is also disclosed a pulley whose design criteria are complimentary to those of the belt to provide a unique combination.